High Performance Electrode Materials From Carbon Nanofibers with Tunable Pore Architectures
Alternative energy sources are required in order to address the depletion and environmental concerns associated with non-renewable energy sources. Uncertainty and sporadic nature of renewable energy sources such as wind and solar, intensify the need of energy storage to supply a secure reliable energy throughout the day. Therefore the energy storage has drawn much attention in the science community. Among various energy storage devices, electrochemical double layer capacitors (EDLCs) also known as supercapacitors have superior properties like excellent performance, rapid charge-discharge, high power densities, along with long cycle life. But EDLCs possess low energy densities (<10 Wh/kg) which limit their use in real world applications. Therefore current research is focused on increasing the energy density of EDLCs while maintaining the existing attractive properties. EDLCs with high energy densities are an emerging field of energy storage. EDLCs rely on the pure electrostatic forces formed at the electrode/electrolyte interface. High surface area carbon and electrolytes with higher working voltage window are the key components to improve the energy density of EDLCs. Novel polyimide (PI) precursors were synthesized and tested for electrochemical performance. The polymers were electrospun to obtain polymer fibers to yield carbon nanofibers (CNFs) upon pyrolysis. The effect of in-situ porogen was analyzed and utilized for preparation of high surface area CNFs. PI with the best performance (6FDA-DABA) was blended with polyacrylonitrile (PAN) and the electrochemical performance was analyzed. PAN and 6FDA-DABA are immiscible and strenuous to incorporate higher amounts of 6FDA-DABA. A better way to incorporate higher amounts of immiscible polymers and obtain uniform morphologies is polymer compatibilization. Before compatibilizing PAN and 6FDA-DABA it was required to study the kinetic behavior of the activation reaction (CO2 oxidation of carbon at 1000 °C) of known compatibilized immiscible polymer blend; which mimics the behavior of PAN and 6FDA-DABA. Therefore a polybenzimidazole (PBI) and 6FDD (6FDA-DAM:DABA) blend was chosen and the kinetic behavior was studied before and after compatibilizing with 2-methylimidazole (2-MI). Chapter 1 interprets the introduction which summarizes the history of supercapacitors, energy storage principles, preparation of high surface area carbon and the use of electrolytes with high working voltage windows. Chapter 2 explicates the novel polyimide precursors for EDLCs. The effect of in-situ porogen was studied and the electrochemical performance of CNFs before and after CO2 activation is described. Chapter 3 describes the behavior of the highest performing polymer (6FDA-DABA) with PAN. Polymer blends comprising PAN and 6FDA-DABA were tested for the electrochemical performance before and after CO2 activation. Chapter 4 elucidates the kinetic behavior associated with CO2 oxidation of CNFs derived from electrospun compatibilized polymer blends of PBI and 6FDD. The rate constants, activation energies were calculated which concluded as the oxidation react becomes faster upon compatibilization of the immiscible polymer blend.
Polyimides, Electrospinning, Carbon fibers, Supercapacitors, Electric double layer
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